Electrical equipment or appliance connected to AC power grid line should satisfy current harmonic standard IEC61000-3-2. With regard to different equipment or applications, IEC61000-3-2 has correspondingly set different current harmonic limits, among which, a Class A limit is for normal electrical equipment, Class B for portable tools and non-professional welding equipment, Class C for lighting equipment, and class D for portable personal computer, monitor and TV.
Existing switching mode power supply technology for realizing power factor correction function, e.g. having the structure as shown in FIG. 1, mainly utilizes passive or active method as introduced below:
PFCmethodAdvantagesDisadvantagesPassiveSimple structure, High loss, heat generated by (resistor)lowest cost.resistor, low PF.Not suitable for medium or high power converter.Not suitable for Class C, Class D equipment.PassiveSimple structure, High loss, low PF.(inductor)low cost.Not suitable for medium or high power converter.Not suitable for Class C, Class D equipment.ActiveHigh PF, low loss.Complex circuit structure.(boost)Suitable for low, medium More parts.and high power converters.High cost.Suitable for Class C, Large size.Class D equipment.
Among the above, active PFC (Boost) is the best power factor correction method in terms of performance.
As shown in FIG. 2, a conventional converter with active PFC comprises an AC-DC rectification circuit, a boost converter and a DC-DC converter. In the practical example shown in FIG. 3, the DC-DC converter is of the half bridge type, and a full bridge or BUCK converter may also apply.
As shown in FIG. 3, the boost converter and the half bridge DC-DC converter operate independently, and are controlled and driven by individual PWM control units. The operating principle is: first boost switching component Q1, inductor L1, diode D2, capacitor C1 and C2 are constructed as the boost converter:
1) Q1 turns on: rectified input voltage on C1 applied on L1, energy stored in L1;
2) Q1 turns off: induced voltage on L1 and voltage on C1 superimpose to charge C2, voltage on C2 always being higher than input instant voltage, so boost circuit operates.
The duty of Q1 is controlled by PWM control unit which senses and feedbacks PFC output (C2 voltage, normally designed to 380V˜400V), and then generates a PWM driving signal by conventional automatic control theory.
The above conventional boost circuit is capable of realizing power factor correction, to meet IEC61000-3-2 requirement, but it has below demerits:
1. Need individual PWM control unit to improve power factor.
2. Need power supply circuit for PFC PWM control circuit.
3. Need independent boost switching components (FETs and diodes) and PFC current sensing resistor R1.
4. Need more PCB space, which is difficult in mechanical and layout design.
5. More components count, high cost.
6. Boost switching component Q1 operates at hard switching mode, results in high loss, poor EMI noise.